Uranium Geology

Origin of Wyoming’s Uranium

Most of Wyoming’s uranium deposits are hosted in medium- to coarse-grained sandstones and similar rocks of Paleocene and Eocene age.
They mostly occur in the Wasatch, Wind River, and Battle Spring formations.
These host rocks are about 40 million to 55 million years old, but the uranium ore deposits contained in them are much younger.

In northeastern Wyoming, uranium host rocks are also found in the lower Cretaceous Lakota and Fall River formations (also
known as the Inyan Kara Group), which are approximately 100 million to 138 million years old. Host rock lithologies include sandstone, arkose,
and conglomerate.

The uranium minerals found in the host rock were leached from their original source rock and
deposited into the host rock as they precipitated out of solution. The solvent, as well as the
transport mechanism, was oxygen-rich surface and groundwater. One proposed source for uranium
ore deposits in Wyoming is Precambrian granitic rocks such as those of the Granite
Mountains in the central part of the state. Uranium occurs as a minor element in minerals
within these igneous rocks. Erosion has removed such substantial amounts of igneous material
from the Granite Mountains, and to such an extent that many geologists believe enough uranium has been removed from
those mountains to account for the ore deposits in the nearby basins.

Another potential source for uranium in Wyoming is Eocene, Oligocene, and younger tuffs
(volcanic ash-rich material). The tuffaceous beds were deposited beginning about 50 million
years ago, forming such rock units as the Wagon Bed and White River formations and their age
equivalents. Volcanism, resulting from molten rock or magma near the surface of the earth, was
widespread throughout much of the western United States as well as northwestern Wyoming, and
occurred periodically for some 40 million years.

At times, volcanic ash blanketed all but the highest peaks and highlands of the state. That
volcanism was the most likely source of tuffaceous beds such as those in the White River Formation.
Erosion that occurred more than millions of years since has removed most of that material, leaving
behind characteristic bluffs such as those in the photo to the left of Shirley Basin in southeastern
Wyoming and at numerous other locations in the state, including Pumpkin Buttes in northeastern
Wyoming (photo on the bottom right).

The White River Formation is exposed in several Wyoming basins and forms broad, near-horizontal
surfaces. As it erodes, it often forms prominent ridges and badland topography, which is
characterized by gullies, steep ridges, and sparse vegetation resulting from severe erosion.